Storage screen for direct-viewing storage tube



Oct. 28, 1958 N. J. KODA 11m. 2,358,463

' STORAGE SCREEN FOR DIRECT-VIEWING STORAGE TUBE Filed July 1, 1955 2Sheets-Sheet 1 3 LLA in Oct. 28, 1958 'N. J. KODA EI'AL 2,858,

STORAGE SCREEN FOR DIRECT-VIEWING STORAGE TUBE 7 Filed July 1; 1955 2Sheets-Sheet 2 hwy/w): f/m/zy 7144/74, .ZZZJQ 4 0500 42' 4 001,

Unitfid tates Pa nt STORAGE SCREEN FOR DIRECT-VIEWING STORAGE TUBE NobuoJ. Koda, Culver City, and Sidney T. Smith, Pacific Palisades, Calif.,assigno'rs to Hughes Aircraft Company, Culver City, Calif., acorporation of Delaware Application July 1, 1955, Serial No. 519,384

3 Claims. (Cl. 313-68) viewing half-tone storage device disclosed in acopending application, now Patent No. 2,790,929 entitled Direct-ViewingHalf-Tone Storage Device, issued to E. E. Herman and G. F. Smith onApril 30, 1957, and assigned to the same 'assignee as the presentinvention. In the Smith et al. patent, a direct-viewing half-tonestorage device is disclosed which incorporates a storage screencomprising an electroformed nickel screen having patches of dielectricmaterial of uniform thickness distributed uniformly over approximately50% of the exposed surface on one side of the meshes thereof. In theoperation of a tube of this type, a charge pattern is produced on thestorage screen which, in turn, controls the flow of flood electrons to aviewing screen to produce a visual presentation of the charge pattern.

It is highly desirable that individual interstices or holes in thestorage screen have uniform control over the flow 0f flood electronsover the entire storage screen. The advantage of this is that a storageelement of information on the storage screen can be of the same order ofmagnitude as an individual hole. On the other hand, if individual holesin the storage screen have random c ntrol with an average near thedesired value, a storage element of information on the storage screenmust contain approximately at least 10 individual holes or controlelements to provide a true representation of the desired averagecontrol. Also, in conjunction with the above, it is desirable that theindividual interstices of storage screen have a uniform extended cut-offcharacteristic. As disclosed'in the Herman et a1. patent, an.

extended cut-oif characteristic was achieved by covering only a portionof the meshes of the screen with dielectric material. This extendedcut-ofi characteristic, however, was not uniform for each interstice ofthe storage screen as is the storage screen of the presentcase.

In accordance with the present invention, a process is disclosed forfabricating a storage screen wherein the dielectric material is disposedsymmetrically about each interstice which may be circular. or square.When this condition exists, each interstice of the screen exerts thesame control over the flow of flood electrons. In the preferredembodiment of this screen, a thin layer of dielectric material isdisposed ,on the central portion of each mesh of an electroformedscreen. An advantage of having the layer of dielectric material disposedon the meshes of the conductive screen in this manner is that theerasure speed is increased, i. e., the storage surface can be charged ina negative direction more efliciently. To erase stored images or toprovide a negative background for good contrast the dielectric materialis charged storage screens.

negatively with low velocity electrons having a secondary emission ratioless than unity. The electric field from the viewing screen penetratesthe interstices of the storage screen and influences the electric fieldimmediately adjacent the layer of dielectric storage material. This isparticularly true for highly transparent, coarse During the erasureprocess this field from the 'viewing screen causes some ofthe-low-velocity flood electrons to be deflected away from the storagesurface, to fall into the interstices, and to ultimately strike theviewing screen. This action results in a loss of erasure electrons andcontributes to the objectional bright flash on the viewing screen duringerasure. In the case of the present invention the layer of dielectricmaterial, due to its central location, is shielded by a maximum amountfrom the influence of the viewing screen field. This would eliminate theneed for lowering the potential applied to the viewing screen duringerasing intervals.

The process for fabricating a storage screen of this type comprisesfirst evaporating a dielectric material over one side of anelectroformed screen without depositing any dielectric material withinthe interstices thereof. This may be accomplished, for example, by theuse of a material known as photo resist in a manner hereinafterexplained. After the dielectric material has been evaporated on thescreen, the meshes of the screen are then increased to the desired widthby plating additional metal on the exposed conductive portions of thesurfaces thereof.

It is therefore an object of the invention to provide a process forfabricating an improved storage screen adaptable for use in adirect-viewing storage device.

Another object of the invention is to provide a storage screen capableof being more efliciently erased and having uniform controlcharacteristics in each interstice of its entire area.

Still another object of the present invention is to provide a processfor fabricating a storage screen having a thin layer of dielectricmaterial disposed symmetrically over only a portion of the exposedsurface on one side of the meshes of an electroformed screen.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, Will be better understoodfrom the following description considered in connection with theaccompanying drawings in which an embodiment of the invention isillustrated by way of example. It is to be expressly understood,however, that the drawings are for the purpose of illustration anddescription only, and are not intended as a definition of the limits ofthe invention.

Figs. 1 and 2 are plan and cross-sectional views, respectively, of anenlarged portion of the storage screen of the present invention;

Figs. 3 and 4 are plan and cross-sectional views, respectively, of anenlarged portion of an electroformed screen; and Figs. 5 to 10illustrate steps in the process of fabricating the storage screen shownin Figs. 1 and 2.

Referring now to the drawings, Figs. 1 and 2 show plan andcross-sectional views, respectively, of an embodiment of the storagescreen of the present invention. More particularly, the storage screencomprises a metallic screen 10 having of the order of 250 meshes perinch, a thickness of 0.001 inch, and a transparency of approximately40%. Along the central portion of the meshes of screen 10 is disposed alayer 12 of dielectric material such as, for example, magnesiumfluoride. Thus, layer 12 has the configuration of a dielectric screenthat is disposed in register with the metallic screen 10. The width ofthe meshes forming the dielectric screen, however, is approximately onlyone-half the width of the meshes of screen 10. The thickness of thelayer 12 of Patented -oer. 28,1958

dielectric, material is not critical and may, for example, be from 1 tomicronsthick.

In accordance with the present invention, the storage screen describedabove is fabricated by commencing with an electroform'ed nickelscreen14('Fig. 2) having the requisite number of meshes per inch and a thicknessof the order of 0.0006 inch; The width, of the meshes of screen 14,however, is made to be substantially equal to that of the dielectricscreen, formed, by the layer12. A plan and a cross-sectional view of anenlargedv portion of the nickel screen14= are shown in Figs. 3 and 4,respectively. The nickel screen 14 is stretched taut in a jig 16 asshown in Fig. 6.

The first principal step in fabricating the storage screen of thepresent invention, is to dispose the layer" 12 of dielectric material onone side of the meshes of the nickel screen 14 without depositing anydielectric material on the inner surface of the interstices thereof.Conventional evaporation techniques have been found to be unsatisfactoryas some molecules of the dielectric material are deposited within theinterstices of the nickel screen 14 which result in non-uniformities inthe finished storage screen. This step, however, may be accomplished byfirst coating the entire surface of the nickel screen 14, as shown inFig. 5, with a layer 18 of photo resist in the absence of light. Photoresist may comprise, for example, a photosensitive lacquer or gelatinthat is characterized by the fact that it is hardened by exposure tolight. Prior to this, it maybe dissolved in a liquid such as water oralcohol, depending on the type of photo resist employed. Also, the photoresist that is hardened by exposure to light may be removed by raisingit to an appropriate temperature determined by its composition or bydissolving it in an appropriate chemical solution.

The portions of the layer 18 of photo resist on one side of the nickelscreen 14 and on the inner surface of the interstices thereof areexposed to light thereby hardening these portions. This exposure may bemade by projecting light towards one side of the coated screen 14 at aslight angle with the normal thereto in the manner shown in Fig. 6. Toaccomplish this, several sources of light may be employed oralternatively, the jig 16 in which the coated screen 14 is mounted maybe rotated while being exposed so that light will be projected over theentire inner periphery of each interstice of the coated screen 14. Theportions of the layer 18 of photo resist exposed to the light will behardened. These portions are indicated by theshaded areas 20 of Fig. 7.The unexposed portions of layer 18 are indicated by the dotted areas 22in the same figure.

After exposure to the light, the layer 18 of photo resist is thendeveloped, i. e., the coated screen 14 is immersed in a liquid capableof dissolving the unhardened portions. Thus, the dotted areas 22 oflayer 18 are washed away leaving only the hardened portions 20 as shownin Fig. 8. After this step of the process, only one side of the meshesof the nickel screen 14 remains uncoated. The thin layer 12 ofdielectric material such as, for example, magnesium fluoride, may now beevaporated on the uncoated side of the meshes of the nickel screen 14.When magnesium fluoride is used, it is desirable to heat the screen 14to a temperature of the order of 150 to 300 C. in order to get a goodbond between the magnesium fluoride and the nickel. In that the photoresist layer 18 has a finite thickness, some dielectric material will bedeposited on the exposed areas of the remaining hardened portions 20 asshown in Fig. 9;

After evaporation of the layer 12 of dielectric material, the nickelscreen 14 is placed in an appropriate medium and heated to a temperaturenecessary to burn the hardened portion 20 of the photo resist layer 18off of the screen 14 or in an appropriate chemical solution to dissolvethe hardened photo resist away. The actual composition of the medium orchemical solution and the necessary temperature will depend on the typeof photo resist employed in the process. As mentioned above, a portionof the layer 12 of dielectric material will have been disposed on top ofthe hardened portion 20 of photo resist layer 18 which has now beenburned away. This portion of the layer 12 will thus be left unsupported.Thence, in View of the fact that the layer 12 of dielectric material isonly from 1 to 10 microns thick, the unsupported portions thereof may bebroken away by agitating the screen 14 in Water or other liquid, or byblowing air over and through the screen 14. After this step has beencompleted, the nickel screen 14 has the layer 12 of dielectric materialdisposed on only one side of the meshes thereof as shown in Fig. 10.

A coating 24 of metal (see- Figs. 1 and 2) is now electroplated on thenickel screen 14 to increase the area of conductive surfacesurroundingthe dielectric. The extent or amount of this plating generallyrepresents a compromise between the percentage of conductive surfacedesired relative to. the area of the layer 12 of dielectric material andthe ultimate transparency of the storage screen. In the event that it isdesired to plate copper on the exposed portions of nickel screen 14, thenickelv screen 14 supported by the jig-16 is immersed in a saturatedsolution of electrolyte such as, for example, copper sulfate with thelayer 12 facing and parallel to an anode. A small direct-current voltageof the order of 1 volt is developed between the anode and the nickelscreen 14. For a screen of the order of 5 inches in diameter, one

' volt will cause a current of-the order of 3 amperes to flow. Duringthe plating operation, the nickel screen 14 is periodically rinsed withwater and rotated in the plane parallel to the anode. The electrolytemay also be agitated or stirred to improve the uniformity of theplating. The plating operation is continued until the desired increasein the width of the meshes of the nickel screen 14. has been achieved.

What is claimed is:

1. In a direct-viewing storage device, a storage screen comprising aconductive screen, and a continuous layer of dielectric materialdisposed about and spaced substantially uniformly from the periphery ofeach interstice of said screen on one side of the meshes thereof.

2. In a direct-viewing storage device, a storage screen comprising anelcctroformed metallic screen, and a continuous layer of dielectricmaterial having secondary electron emission characteristics disposedabout and substantially uniformly spaced from the periphery of eachinterstice of said screen on one side of the meshes thereof, thethickness of said layer being small relative to the thickness of saidscreen.

3. In a direct-viewing storage device, the storage screen as defined inclaim 2 wherein the area of said continuous layer of dielectric materialis substantially equal to the portion of the area of said meshesremaining exposed on said one side.

References Cited in the file of this patent UNITED STATES PATENTS2,156,435 Schroter et al. May 2, 1939 2,287,415 Bwinett June 23, 1942-2,539,422 Larson Jan. 30, 1951 2,682,501 Teal June 29, 1954 2,685,660Norgaard Aug. 3, 1954 2,754,449 Farnsworth July 10, 1956

